PL/SQL TRIGGER COMPILE ERROR - oracle

Question

create or replace trigger enroll_bef_ins_row
before insert on enrollments
for each row
declare
original number;
seatsremain_already_full exception;
begin
Select seatsremain into original from offering where offerno= :new.offerno;
if original > 0 then
update offering set seatsremain= seatsremain - 1;
dmbs_output.put_line ('Seats available in offering' |offerno| 'have decreased from' |:old.seatsremain| to |:new.seatsremain|);
else if original = 0 then
dbms_output.put_line ('Offering' |offerno| 'is already full!');
raise seatsremain_already_full
end if;
exception
when seatsremain_already_full
raise_application_error (-20001, 'Cannot allow insertion');
end
/
I keep getting a "trigger created with compilation error" message and every time I try to insert in values I get a ORA 04098 - SYSTEM.ENROLL_BEFORE_INS_ROW IS INVALID AND FAILED REVALIDATION message.
My task is to write a trigger that performs the following tasks before a row has been inserted into the enrollments table:
If the seats are available for the particular offering, the trigger should automatically decrease the number of seats for the offering and display the message: seats availabe in offering (offering number) have decreased from (number of seats available prior to insertion) to (number of seats available after insertion). If the number of seats available after the insertion is equal to 0, display message: 'No more seats in offering (offering number should be entered here)
If the seats available for a particular offering were equal to 0 prior the insertion of the row in enrollments, the trigger should display the following:
insertion not allowed
and use the raise_application_error procedure to prevent the execution of the INSERT statement

Answer 1

The concatenation operator is ||, and all statements must end with a semi-colon. There was also the odd apostrophe missing here and there.
Try this:
create or replace trigger enroll_bef_ins_row
before insert on enrollments
for each row
declare
original number;
seatsremain_already_full exception;
begin
Select seatsremain
into original
from offering
where offerno = :new.offerno;
if original > 0 then
update offering
set seatsremain = seatsremain - 1
WHERE OFFERNO = :new.OFFERNO;
dmbs_output.put_line ('Seats available in offering' || :new.offerno ||
'have decreased from' || original ||
' to ' || original-1);
elsif original = 0 then
dbms_output.put_line ('Offering' || offerno ||
' is already full!');
raise seatsremain_already_full;
end if;
exception
when seatsremain_already_full
raise_application_error (-20001, 'Cannot allow insertion');
end;
I suggest that you might want to NOT decrement SEATSREMAIN but instead compute the number of seats remaining by taking OFFERING.SEATSAVAILABLE and substracting the sum of the seats taken - otherwise there's a race condition in there on OFFERING.SEATSREMAIN and you can end up overbooking - but this will probably work for simple cases. In addition, you might want to raise an error if SEATSREMAIN ends up negative.
Share and enjoy.